libunwind.h not found

[helly25]

After adding a file that depends on #include <libunwind.h> the compilation fails with the file not being found. Interestingly if the include is changed to unwind.h then the file is found. Further, an attempt to patch the toolchains to add external/llvm_toolchains_llvm/include to the cxx_builtin_include_directories does not fix the issue. So it is completely unclear (to me) why one file can be found and the other not.